The present invention relates to radio frequency identification (RFID) tags, and in particular, to RFID tags having a ferrite core.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
The quality factor (also referred to as the “Q factor” or “Q”) is a dimensionless parameter that compares the time constant for decay of an oscillating physical system's amplitude to its oscillation period. Equivalently, it compares the frequency at which a system oscillates to the rate at which it dissipates its energy. A higher Q indicates a lower rate of energy dissipation relative to the oscillation frequency, so the oscillations die out more slowly. Regarding RFID circuits, Q is a measure of the “quality” desired in a good tuned circuit or other resonator.
More specifically, when an RFID tag is driven by a sinusoidal drive, its resonant behavior depends strongly on Q. Resonant systems respond to frequencies close to their natural frequency much more strongly than they respond to other frequencies. A RFID tag with a high Q resonates with a greater amplitude (at the resonant frequency) than one with a low Q factor. The amplitude of the resonant response affects the read range. To increase sensitivity and read range, many existing RFID tags have a relatively high Q. As a specific example, the Q of many existing RFID tags is between 10 and 20.
When multiple tags are in close proximity, however, they tend to interact. This interaction changes their resonant frequency of operation. Instead of a single resonance at the desired frequency, this interaction results in multiple resonances at undesirable frequencies. As a result, the tags are not energized and/or data is not successfully exchanged when the tags are in close proximity. Various approaches may be used to mitigate this interaction, but these strategies may reduce the read range. Compensating this reduction in read range by increasing the power output of the reader is possible but not always practical.
Thus, there is a need to read RFID tags in close proximity, and to do so with an acceptable read range.